The present invention relates to a method and apparatus for balancing the electrical parameters in three-phase arc discharge furnaces, particularly also in order to compensate asymmetries in input inductances, by changing the transformer voltage and/or the electrode height positions.
The three arcs of a three-phase arc discharge furnace each have four variables, or parameters, that are significant to operation, namely: the length of the arc, which is essentially proportional to the arc voltage; the arc current; the arc power; and a value which is a function of the arc length and the arc current and is determinative for the amount of wear of the furnace lining from electrical causes. Normally, it is desired to have these variables in symmetry between, relative to a neutral point, the three windings, i.e. the three arcs. Symmetry between the arcs exists whenever the three arc voltages are identical and the three arc currents are identical. This inevitably makes all of the other arc variables symmetrical as well, i.e. the arc power, the wear coefficient and the arc resistance, if one disregards possible influences from harmonics. However, this symmetry is disturbed, in particular, by asymmetries, or inequalities, in the input inductances. Even furnaces having symmetrical input inductances, as can be realized, for example, by so-called triangulated line conduction, again and again exhibit deviations from symmetry in operation, because the heights of the supporting arms are different, for example, due to the electrodes being clamped in at different lengths and because, during the melting of scrap, irregular scrap charges may position the three electrode tips at unequal distances from the melt or charge.
Although it is possible, if the input inductances are asymmetrical, to regulate the electrode currents to be in symmetry by way of adjusting the electrode heights, this then means that different arc voltages, i.e. arc lengths, must be accepted. With an upper limit for arc power and wear coefficient, this means that possibly only one arc attains these limits, while the other two arcs are not operated at their maximum possible output. If, however, one were to regulate for identical arc voltages, then the currents would become asymmetrical. Possibly, then, the full current intensity can be utilized in only one winding while this is not the case in the other two windings.